Active galactic nuclei in the mid-IR
Evolution and contribution to the cosmic infrared background
Max-Planck Institut für extraterrestrische Physik (MPE), Giessenbachstra$\beta$e, Postfach 1312, 85741 Garching, Germany e-mail: email@example.com
2 Dipartimento di Fisica, Università degli Studi “Roma Tre”, via della Vasca Navale 84, 00146 Roma, Italy e-mail: firstname.lastname@example.org
3 Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
4 INAF, Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
5 INAF, Istituto di Radioastronomia (IRA), via Gobetti 101, 40129 Bologna, Italy
Accepted: 30 December 2005
Aims.We study the evolution of the luminosity function (LF) of type-1 and type-2 Active Galactic Nuclei (AGN) in the mid-infrared, derive the contribution of the AGN to the Cosmic InfraRed Background (CIRB) and the expected source counts to be observed by at 24 μm.
Methods.We used a sample of type-1 and type-2 AGN selected at 15m (ISO) and 12m (IRAS), and classified on the basis of their optical spectra. Local spectral templates of type-1 and type-2 AGN have been used to derive the intrinsic 15m luminosities. We adopted an evolving smooth two-power law shape of the LF, whose parameters have been derived using an un-binned maximum likelihood method.
Results.We find that the LF of type-1 AGN is compatible with a pure luminosity evolution () model where . A small flattening of the faint () slope of the LF with increasing redshift is favoured by the data. A similar evolutionary scenario is found for the type-2 population with a rate kL ranging from ~1.8 to 2.6, depending significantly on the adopted mid-infrared spectral energy distribution. For type-2 AGN a flattening of the LF with increasing redshift is suggested by the data, possibly caused by the loss of a fraction of type-2 AGN hidden within the optically classified starburst and normal galaxies. The type-1 AGN contribution to the CIRB at 15m is (4.2–12.1) , while the type-2 AGN contribution is (5.5–11.0) . We expect that will observe, down to a flux limit of mJy, a density of ~1200 deg-2 type-1 and ~1000 deg-2 type-2 optically classified AGN.
Conclusions.AGN evolve in the mid-infrared with a rate similar to the ones found in the optical and X-rays bands. The derived total contribution of the AGN to the CIRB (4–10%) and counts should be considered as lower limits, because of a possible loss of type-2 sources caused by the optical classification.
© ESO, 2006